Semiconductor devices are currently of interest for many applications including light sources for optical communications systems. Such systems, as presently contemplated, use glass transmission lines, commonly termed optical fibers, to optically couple a light source and photodetector. The preferred light source at the present time is a heterostructure semiconductor laser. A laser of this type comprises an active layer, i.e., a region where radiative recombination of electrons and holes occurs which is located between two cladding layers. The latter two layers provide carrier and optical field confinement.
Although many parameters are of interest in evaluating heterostructure lasers for optical communications purposes, one parameter of special interest is the current threshold, that is, the current density at which the semiconductor device begins to emit coherent electromagnetic radiation. It is generally desired that the current threshold be as low as possible to minimize, for example, ohmic heating in the device and power consumption, and to enable the laser to operate at high ambient temperatures.
Accordingly, structures have been devised with the intent of decreasing the current threshold. For example, a three layer double heterostructure laser having a GaAs active layer between Al.sub.x Ga.sub.1-x As cladding layers can have a very high Al concentration in the cladding layers, typically x=0.6 and an active layer which is very thin, typically between 500 Angstroms and 1000 Angstroms. The large compositional change causes a large refractive index discontinuity between the active and cladding layers. This results in a greater amount of the optical field being confined to the active layer and as a consequence, there is a larger overlap between the optical field and the carriers. This increased interaction between the optical radiation and the carriers reduces the current threshold.
Another approach is described in U.S. Pat. No. 3,911,376 issued on Sep. 12, 1972 to Izuo Hayashi. The semiconductor device described has a narrow bandgap, for example, a GaAs active layer between two wider bandgap, for example, AlGaAs layers and additional, for example, AlGaAs layers with an intermediate bandgap between the active layer and the cladding layers. The additional layers form an optical waveguide which increases the amount of optical field energy in the active layer. As the optical confinement is increased, the current threshold decreases. The bandgaps, which are determined by the aluminum concentrations for AlGaAs layers, are selected to ensure carrier confinement in the active layer and to keep as much as possible of the optical field energy in the layers between the cladding layers. This structure, which is commonly termed a separate confinement heterostructure, thus has two pairs of heterojunctions with the inner pair providing carrier confinement and the outer pair providing optical confinement with each layer having uniform AlAs compositions. However, the optical confinement is significantly increased only for thin active layers, i.e., for layers having a thickness less than .lambda. where .lambda. is the wavelength of the radiation as measured in the semiconductor body.
Another semiconductor device potentially of interest for optical communications systems is the AlGaAs graded index waveguide described in U.S. Pat. No. 4,152,044 issued on May 1, 1979. The structure disclosed has a single graded refractive index layer adjacent the active layer and the graded index layer forms an optical waveguide. The resulting structure is asymmetric with respect to the active layer. Due to the asymmetry, the optical field is not effectively coupled to the carriers in the active layer. Hence, no reduction in threshold is obtained. However, the waveguide layer may be used to either narrow the beam of emitted light and thereby increase coupling efficiency between the light source and an optical fiber or to guide, in an integrated optical device, light between a light source and photodetector with the light source and photodetector being formed on a common substrate.